Suppressing non-radiative recombination in metal halide perovskite solar cells by synergistic effect of ferroelasticity

The low fraction of non-radiative recombination established the foundation of metal halide perovskite solar cells. However, the origin of low non-radiative recombination in metal halide perovskite materials is still not well-understood. Herein, we find that the non-radiative recombination in twinning-tetragonal phase methylammonium lead halide (MAPbIxCl3-x) is apparently suppressed by applying an electric field, which leads to a remarkable increase of the open-circuit voltage from 1.12 V to 1.26 V. Possible effects of ionic migration and light soaking on the open-circuit voltage enhancement are excluded experimentally by control experiments. Microscopic and macroscopic characterizations reveal an excellent correlation between the ferroelastic lattice deformation and the suppression of non-radiative recombination. The calculation result suggests the existence of lattice polarization in self-stabilizable deformed domain walls, indicating the charge separation that facilitated by lattice polarization is accountable for the suppressed non-radiative recombination. This work provides an understanding of the excellent performance of metal halide perovskite solar cells.

As shown in Supplementary Fig.6, it is observed that a small deviation of azimuth angle exists between the Bragg spots, the deviation angle of the twinning plane-sets is caused by the mismatched interspacing of t(110) and t(002) plane-sets, which can be calculated by the following formular 1 : The calculated separation angle is ~1°, which is in excellent agreement with the observation from Supplementary Fig.6.
Supplementary Fig.7 shows the d versus W-H slope, as the strain level enhances the MAPbIxCl3-x change the phase from mC-phase to tT-phase. At particular value of strain the dc(100) convert to dt(110) and dt(002). Here the low symmetry phase obtained from stimuli-triggered symmetry breakdown is different from the counterpart of spontaneous phase transition. It indicated that MAPbIxCl3-x has composed of T-phase domains with two orientations and related through a lost symmetry element during symmetry breakdown. This is direct evidence of the formation of twin-domains by the stimuli-triggered symmetry breakdown 2,3 . The result of EQE measurement is presented in Supplementary Fig.9, it is found that the integrated current density obtained from EQE is comparable with the short circuit current density that obtained from JV scan ( Figure 2a). Increased photovoltaic performance that caused by activation effect, the first J-V scan is shown as black lines in Supplementary Fig.10. The solar cell is then activated for 1 cycle (regular method), an increment on Voc can be readily observed from the 2 nd J-V scan (red lines in Supplementary Fig.10). It has been reported that the ion mitigation has strong correlation with the J-V hysteresis 4,5 .
As shown in Supplementary Fig.12, forward and reverse scans were plotted at fresh and activated status, respectively. where Ai is the amplitude of each component, and τi is the corresponding lifetime. As summarized in Supplementary Table3, the tr-PL shows multiple-decay components, including a fast initial drop of intensity (within tens of ns) followed by a slow (hundreds of ns) relaxation process, carrier lifetime of the two major components is distinctly different (a few ns vs tens of ns). The multi-decay process implies a heterogeneous distribution of carriers with various lifetime values, they can be considered as two subpopulations of carriers with different recombination rates. The percentage of the long (short) lifetime carriers increased (decreased) after activation, indicating the recombination route is slightly varied after the activation.
Supplementary Fig.17: XRD spectrum of tT-phase solar cells that have been activated for various cycles.
Supplementary Fig.18: XRD pattern of (a) tT-phase and (b) T-phase solar cells before and after electric activation.
The correlation between the crystallographic variation and photovoltaic performance was developed to understand the electric activation effect. As shown in Supplementary Fig.18 It is found that the electric field can activate the solar cell without the illumination ( Supplementary Fig.20a). On the contrary, no sign of Voc or PCE increment can be observed only by illumination (Supplementary Fig.20b). The observation indicates that, even under illumination, if the device is short circuited and hence no electric field is applied , the device cannot be activated. The observation revealed that the electric field applied on MAPbIxCl3-x is the trigger of activation effect, instead of the illumination.